Despite reasonable clinical experience with mixtures of L-amino acids in the management of tyrosinemia, L-amino acids which are the source of approximately 75% of dietary protein equivalent in this condition, have not been systematically studied to determine their efficacy in supporting longterm growth and in maintaining normal plasma amino acid profiles. This study is an attempt to evaluate the ability of the L-amino acid mixture in Tyromex, a product in the RMFS, to support normal nutritional status of infants and toddlers.